As people demand more on video/image resolution, video/image contents occupy more and more data transmission bandwidth and storage capacity. How to further increase the video/image compression ratio has become a serious challenge. Especially in intra-frame video/image encoding, the compression efficiency cannot be effectively improved due to lack of inter-frame image reference.
In intra-frame video/Image encoding, intra-frame prediction is adopted to remove redundant spatial information. Before performing intra-frame prediction, the reference pixel points of an intra-frame block to be predicted need to be filtered to eliminate the step effect in the reference pixels. The step in the reference pixel points will lead to a significant directional boundary in the intra-frame block to be predicted, affecting the efficiency and effect of the intra-frame prediction.
At present, the following methods are used for the intra-frame reference pixel points:
Conducting weak filtering on reference pixel points by using a filter with a length of 3 (i.e., a 3-tap filter) and the filter coefficient of [¼, 2/4, ¼], which can only remove small step and noise in the reference pixel points;
The pixel value of each reference pixel point is linearly interpolated by using the pixel value of reference pixel points at the outermost edge (i.e. edge reference pixel points). In this way, the reference pixel values are greatly modified. Therefore, at present, this method only applies to cases where the overall reference pixel points are very flat.
Another way is to combine the above two methods, that is, to determine to use a weak filter with a length of 3 (i.e., a 3-tap filter) and the filter coefficient of [¼, 2/4, ¼] or a linear interpolation filter for filtering of various reference pixel points in a reference pixel group of an intra-frame block to be predicted according to the overall flatness of the reference pixel points (the degree of flatness is determined by the difference between the end edge reference pixel points in a reference pixel group). No matter which method is selected, the reference pixel points in a reference pixel group are uniformly filtered by the selected filtering method, that is, the filtering grades for the reference pixel points in a reference pixel group are identical.
It can be seen from the above analysis that, in the current intra-frame video/image encoding process, the adaptive mechanism of filtering processing of various reference pixel points in a reference pixel group of an intra-frame block to be predicted is too simple. The reference pixel points in a reference pixel group are uniformly filtered by using a fixed filtering grade, resulting in poor filtering flexibility, filtering adaptivity and filtering effect for the intra-frame reference pixel points.